1. Field of the Invention
The present invention relates to a delta sigma modulator, specifically to a delta sigma modulator capable of switching an order thereof into an optimum order in relation to a sampling frequency.
2. Description of the Related Art
At present, many cellular phones, PDAs (Personal Digital Assistance), portable music reproducers, and so forth use a DA converter. As this sort of DA converter is widely known the DA converter that incorporates a delta sigma modulator. This DA converter furnished with the delta sigma modulator executes a quantization with fewer bits such as one-bit quantization by means of the over-sampling circuit and noise shaper, and thereby reduces aliasing and quantization noises, and noises in the low frequency band.
Now, in the delta sigma modulator used in the noise shaper, there exists a unique relation between the SN ratio and the order of the delta sigma modulator in correspondence with each of the sampling frequencies as an example, as illustrated in FIG. 9. In the drawing, X-axis represents the order of the delta sigma modulator, and Y-axis represents the SN ratio.
According to this graph, when the sampling frequency is 8 kHz, and when the order of the delta sigma modulator is the third order, the SN ratio becomes the maximum at about 57 dB; when the order increases to the fourth or fifth order, the SN ratio decreases to 55 dB or 40 dB.
In contrast this, when the sampling frequency is 16 kHz, and when the order of the delta sigma modulator is the second order, the SN ratio is about 62 dB; when the order becomes the third or fourth, the SN ratio increases to 72 dB or 73 dB; and when the order is the fifth, the SN ratio decreases to about 69 dB.
Further, when the sampling frequency is 32 kHz, and when the order of the delta sigma modulator is the second order, the SN ratio is 80 dB; when the order is the third, the SN ratio increases; and when the order is the fourth or fifth, the SN ratio reaches the peak at about 90 dB.
As it is clear from the above, the SN ratio will increase or decrease depending on the sampling frequency when the order increases. The delta sigma modulator with a higher order does not necessarily produce a higher SN ratio. Here, FIG. 9 only gives one example, and such a disposition as shown in FIG. 9 does not always appear.
Conventionally, the delta sigma modulator used in the DA converter is designed on the assumption of a specific sampling frequency; accordingly, the order of the delta sigma modulator is fixed, and it could not be changed freely. However in recent years, the mobile telephones can be used in the voice mode on speech communications, or they can be used in the audio mode that outputs a piece of music downloaded; there increases a possibility of using the DA converter with different sampling frequencies.
When the DA converter is used in the audio band (20 kHz), to maximize the SN ratio in connection with the sampling frequency (44.1 kHz) is to select the delta sigma modulator of the fourth or fifth order as the optimum order. However, using this delta sigma modulator with the lower sampling frequency (8 kHz) that handles the voice will deteriorate the SN ratio, in comparison to the delta sigma modulator of the second or third order.
In reverse, when the modulator is used with the lower sampling frequency (8 kHz), the delta sigma modulator of the third order is to be selected in view of the optimum SN ratio; and, when the delta sigma modulator of the third order is used with the higher sampling frequency (44.1 kHz) for the audio band, the SN ratio will deteriorate in comparison to the delta sigma modulator of the fourth or fifth order.
In this manner, there is a specific relation between the sampling frequency and the optimum order of the delta sigma modulator. For example, it is clear that when the sampling frequency is 8 kHz, 16 kHz, 32 kHz, 44.1 kHz, or 48 kHz, the optimum order is the second, fourth, fifth, fourth (or fifth), or fifth, respectively. This is shown in FIG. 6.
In order to always set an optimum order in correspondence with variations of the sampling frequencies, it is conceivable to prepare the delta sigma modulators of the first order to the n-th order in advance, and to make them selectable by switching. However, such a design will enlarge the circuit scale only to raise the cost and increase waste. As to the switching operation of the order, it is extremely annoying to manually switch the order of the modulator at each time, accompanied with the switching of the sampling frequencies, which will create malfunctions.